Optical materials, such as glass and crystal, exhibit many different properties affecting both the optical performance and the application suitability. An optical material's index of refraction determines the velocity of light through the material. An optical material's dispersion describes the dependence of its index of refraction upon wavelength. An optical material's hardness describes its mechanical resistance to physical contact. An optical material's chemical resistance describes its resistance to moisture and chemical attack. An optical material's electro-optic properties describe how some characteristics, such as form and refractive index, vary with changes to the local electro-magnetic field.
An imaging system may include, e.g., lenses, mirrors, windows, filters, crystals, and so forth. Light passes through the optics and interacts with each material in a manner unique to that material. An imaging system typically transposes spatial information from an object in the field of view, to an image at an image field. One or both of the object plane and image plane may be inside or outside the system. For example, a camera may be described as having an object field outside the system and an image plane inside the system. A projector, however, may be described as having an object field inside the system and an image plane outside the system.
A subset of optical materials, most typically crystal materials, exhibit birefringence. In a birefringent material, the refractive index depends not only on wavelength, but also on the polarization state of the light. A birefringent material will refract light of one polarization state differently than it will refract light of another polarization state. If a lens is made using a birefringent material, and if the incident light on the birefringent lens is randomly polarized, then the birefringent lens will produce two images, superimposed, where one image is slightly larger than the other. On the optical axis, the features in the two images will be concentric. However, further from the optical axis, there may appear blurring in the fine detail as the lens exhibiting slightly different powers, or magnifications, for the two polarization-dependent beam paths.